1. Field of the Invention
The invention relates to devices for interconnecting networks in general and, in particular, to the type of devices termed switches.
2. Prior Art
The use of devices for interconnecting communications networks are well known in the prior art. The devices are known by different names including gateways, bridges, hubs, routers, switches, etc. In spite of what they are called, these devices provide interconnection features which enable a device on one communications network, such as a LAN, to communicate with another device on another LAN. Examples of prior art interconnecting devices will now be described.
U.S. Pat. No. 5,088,090 discloses a bridge for interconnecting LANs. Based upon information contained in a frame header, the bridge uses Source Routing Technique or Transparent Bridging Technique to forward the frame from one LAN to another. The routing technique used in the bridge is dependent on the one used by the originating node. For example, if the originating node is a Source Routing Node, the bridge uses
Source Routing Technique to forward the frame. Likewise, if the originating node is a transparent bridged node, the bridge uses Transparent Bridging Technique to forward the frame.
U.S. Pat. No. 5,280,480 describes a bridge for routing frames from Source Routing nodes or Transparent Bridging nodes. The patent uses Bridge Protocol Data Unit (BPDU) Frame to set the bridge in a forwarding or blocking state and uses information in the header of Non-BPDU frames to forward the frames using the Source Routing technique or Transparent Bridging technique.
Even though the above bridge devices work well for their intended purposes, they are basically two port devices and offer limited interconnecting options. Arguably, several bridges can be configured to form complex networks of interconnected LANs. However, such networks are usually costly, difficult to configure and difficult to manage.
In addition, it is believed that as LANs become more congested (due to network expansion, attachment of higher speed devices, etc.), there is a need to provide faster and more efficient interconnecting devices.
U.S. Pat. No. 5,274,631 describes a switching system for interconnecting ethernet LANs. Each of the ports are connected through a packet processor to a switch fabric. A system processor is also connected to the switch fabric. When a packet processor receives a packet from the port to which it is connected and if the packet processor knows the destination port, the packet is routed through the switch fabric to the port. If the destination port is unknown to the packet processor, the packet is routed through the switch fabric to the system processor to identify the destination port. In this switching system, routing is done based on the destination addresses. As a consequence, large look-up tables correlating addresses with ports are required at each port. In addition, this switch can only be used to interconnect ethernet LANs. Therefore, there is a need for a switching system to interconnect Token Ring LANs. The present invention described below provides such a switching system.
Other IEEE documents including 8802-S and P802.5R describe devices in which the port determination is made at different functional units within the system. These devices use multiple look-up tables and multiple process, steps to select ports through which a frame is to be routed.